Competence for genetic transformation in Streptococcus pneumoniae: organization of a regulatory locus with homology to two lactococcin A secretion genes

Abstract

Regulation of competence for genetic transformation in Streptococcus pneumoniae involves the comAB locus and a small extracellular protein, the competence factor (CF). The comA or comB mutations block both spontaneous competence induction and elaboration of CF, yet permit induction of competence by added CF and subsequent transformation at normal levels. Sequence and genetic studies showed that the com locus comprised the comA and comB genes, encoding 77- and 50-kDa proteins, respectively, and demonstrated that they were closely flanked by genes and DNA not required for competence regulation. In-frame deletion of comA demonstrated that the translation product of this gene is required for normal competence regulation; deletion-replacement mutations showed that no com gene lay in the 0.2-kb gap between comB and purC or within 2.5 kb upstream from comA. Strong sequence similarities (51-59% identities) showed that ComA and the proteins, PdcD and LcnC, which act in the secretion of pediocin A-1 and lactococcin A, respectively, form a subfamily within the large ABC-transporter protein family. ComB was found to be homologous to a single known protein, LcnD, required for secretion of the peptide antibiotic lactococcin A. Thus, the comAB locus displays homology to two lactococcin A secretion genes, but is devoid of additional linked com genes. The results suggest that the mechanism for CF production is similar to that for the small peptide bacteriocins, lactococcin A and pediocin A-1, but that its genetic organization is unusual in being split into at least two separate operons.